Method and electrolyte for electroplating brass



United States Patent Office US. Cl. 204-44 9 Claims ABSTRACT OF THE DISCLOSURE An electrolyte and process for electrodeposition of brass from an aqueous alkaline cyanide bath containing zinc cyanide, copper cyanide and an indium compound such as indium chloride. The process gives a brass deposit of improved corrosion resistance.

This invention relates to an electroplating process. In particular, it relates to the art of electrodeposition from a cyanide bath of a brass alloy with more desirable properties than can be obtained by the use of prior art processes. By the use of the present invention, a deposit is obtained which imparts greater protection to corrosion of the base metal than was previously possible.

The invention is based-on the discovery of a new composition of plating bath which, while substantially the same as prior compositions with respect to the major ingredients, differs therefrom by including certain additives which produce extraordinary beneficial effects.

It has been discovered that by making an addition of a small amount of the metal ion indium to a cyanide brass plating bath, extraordinary beneficial results are obtained. When a conventional brass bath is modified by the addition of a soluble compound of indium, the alloy which is electrodeposited on the metal surface affords much greater protection than the deposited metal from conventional cyanide brass baths. This is especially true when the brass is one of a series of successive metal deposits on a base metal, such as steel.

The protection is due, in part, to the increased resistance to corrosion of the base metal. This property of increased corrosion resistance can be demonstrated by various tests. One such test involves the use of an accelerated corrosion test of steel panels which are plated successively with copper, brass, nickel and chromium. One set of panels is prepared using brass from a conventional brass plating bath while another set of panels is prepared using brass from a bath of the present invention. The panels are then placed in a press with a steel ball placed at the back of the panel. Pressure is then applied to the ball in order to distort the panel so that an extrusion of two tenths of an inch will result and the plated surface will become ruptured under the stress. These panels are then subjected to the conventional accelerated corrosion test known as the Corrodkote test. From the beginning, observations are then made every hour to determine when red rust will appear in the cracks of the plated metal on the extruded portion of the panels, or when blisters will appear on the flat unstressed portion of the plated panel. If either of these conditions occur, the panel is considered a failure.

The increased protection from corrosion of the base metal when using brass deposited from the baths containing indium is readily demonstrated. Some of the panels which are plated using the conventional brass bath will begin to show red rust in the crevices of the plated surface on the extrusions within one hour, and most of the panels so plated have usually failed within 3,440,152. Patented Apr. 22, 1969 sixteen hours. On the contrary, the panels plated from the brass bath containing indium in accordance with the present invention usually show no red rust onthe panels and in some cases at least until after ninety hours of expos'ur'e to accelerated corrosion. Another observation is that the flat portion of the panel shows little or no blistering'on' the panels which are plated with the brass bath of the present invention, while the panels plated with the conventional brass bath will show blistering as early as sixteen hours after the beginning of the test.

Another advantage of the invention resides in the fact that the brass alloy deposited from the baths of this invention reduces the migration of copper strike into said brass alloy when the copper strike is used preceding a subsequent nickel layer. This then enhances the adhesion of the nickel layer to the other underlying layers.

According to this invention, a conventional cyanide brass plating bath can be modified by the addition of small amounts of the indium ions.

A typical example of a bath composition is as follows:

Zinc cyanide ounces per gallon 3.8-4.1 Copper cyanide do 2.5-2.7 Free cyanide do 7.5-8.0 Caustic soda do 2.5-5.5 Indium chloride grams per gal 0.001-0.0l

When the bath set forth above is used with conventional brass anodes containing zinc between the range of 55% to 60% and copper in the range of 40% to 45% and an electrical current density of 30 amperes to 40 amperes per square foot, the resulting brass deposit will possess the desired characteristics described above.

However, the bath composition is not necessarily limited to the precise limitations given above, for the invention may be successfully practiced with cyanide brass plating baths in which the zinc to copper ratio may vary to a considerable degree, and as long as the indium ions (or equivalent metal ions) are present in the ranges specified above, the resulting plate will impart improved protection to the base metal. Therefore, in its broader aspects, the invention is operative when the ingredients are in the following wider ranges:

Zinc (calculated as metal) oz. per gal Copper (calculated as metal) do Total zinc and copper (calculated as metal do 2.7-8.5 Ratio Cu/Zn 45/55-40/60 Indium (or equivalent) (calculated as metal) "grams per gal 0.0003-0.2S Free cyanide (calculated as NaCN) oz. per gal 6.5-l0.0 Caustic (calculated as NaOH) do 1.5-6.5

In the composition ranges given above, it should be understood that the ranges of ingredients are listed for the purpose of supplying an operating range wherein the electrodeposited brass alloy has good corrosion resistance and an acceptable appearance. This does not necessarily imply that the invention is inoperative beyond such limits.

Although the ranges of ingredients can be varied within wide limits as indicated in the preceding formula, it is preferred that the bath composition should be maintained within the ranges set forth in the earlier typical example. In preparing a new bath, it is recommended that it should be carbon treated in accordance with standard procedure, filtered and dummied for several hours before its use for commercial plating. A bath should be operated using bagged anodes, continuous filtration, air agitation and preferably with interrupted current. The desirable temperature range of operation for the bath is between 78 F. and F. The recommended current density is Zinc (calculated as metal) oz. per gal..- 1.5-5.0 Copper (calculated as metal) do 1.2-3.5 Total zinc and copper (colculated as metal) oz. per gal 2.7-8.5 Ratio Cu/7n 45/ 55-40/ 60 Indium (calculated as metal) grams per gal 00003-025 Free cyanide (calculated as NaCN) oz. per gal 6.5-l0.0 Caustic (calculated as NaOH) do 1.5-6.5

2. A bath according to claim 1 in which the constituents are within the following ranges:

Zinc cyanide ounces per gallon 3.8 to 4.1 Copper cyanide do 2.5 to 2.7 Free cyanide -do 7.5 to 8.0 Caustic soda do 2.5 to 5.5 Indium compound ..grams per gal 0.001 to 0.01

3. A bath according to claim 2 in which the indium compound is indium chloride.

4. An aqueous alkaline metal plating bath comprising 1.2 to 3.5 oz. per gal. of copper as copper cyanide, 1.5 to 5.0 oz. per gal. of zinc as zinc cyanide, and indium ions in an amount sufficient to provide an electrodeposit having improved corrosion resistance.

5. In the process of electrodepositing brass from an aqueous alkaline cyanide bath comprising zinc and cop per in solution, the improvement in said process of having indium present in solution in an amount sufficient to give a deposit of improved corrosion resistance.

6. A process according to claim 5 in which the bath is within the following ranges:

Zinc cyanide ounces per gallon 3.8 to 4.1 Copper cyanide do 2.5 to 2.7 Free cyanide do 7.5 to 8.0 Caustic soda do-. 2.5 to 5.5

Indium compound (calculated as metal) grams per gaL- 0.001 to 0.01

7. A process according to claim 6 in which the indium compound is indium chloride.

8. A process according to claim 5 in which said indium is present in an amount calculated as grams of metal per gallon between 0.0003 and 0.25.

9. A process according to claim 5 in which the bath is within the following ranges:

Zinc (calculated as metal) ounces per gallon 1 55.0 Copper (calculated as metal) do 1 23.5 Total zinc and copper (calculated as metal) ounces per gallon 2.7-8.5

Ratio Cu/Zn /55-40/60 Indium (calculated as metal) grams per gallon Free cyanide (calculated as NaCN) ounces per gallon 6.5-10.0 Caustic (calculated as NaOH) do 1.5-6.5

References Cited UNITED STATES PATENTS 2,182,381 12/1939 Hensel et al. -157.5 XR 2,989,448 6/1961 France 204-44 3,021,266 2/ 1962 Ostrow et al. 20444 OTHER REFERENCES Lowenheim, Frederick A.: Modern Electroplating, Second Edition; p. 6, 1963.

ROBERT K. MIHALEK, Primary Examiner.

G. L. KAPLAN, Assistant Examiner.

US. Cl. X.R. 75-157.5 

